1. Field of the Invention
The present invention relates to an ink composition suitably used for inkjet recording, an inkjet recording method, and a printed material employing same; furthermore, it relates to a lithographic printing plate obtained using the ink composition, and a process for producing a lithographic printing plate. More particularly, it relates to an ink composition suitable for inkjet recording that cures with high sensitivity upon exposure to radiation, can form a high quality image, and has good storage stability, an inkjet recording method, a printed material employing same, a lithographic printing plate obtained using the ink composition, and a process for producing a lithographic printing plate.
2. Description of the Related Art
With regard to an image recording method for forming an image on a recording medium such as paper based on an image data signal, there are an electrophotographic system, sublimation type and melt type thermal transfer systems, an inkjet system, etc. In the electrophotographic system, a process of forming an electrostatic latent image on a photosensitive drum by electrically charging and exposing is required, and the system is complicated; as a result, there is the problem that the production cost is high. With regard to the thermal transfer system, although the equipment is inexpensive, due to the use of an ink ribbon there is the problem that the running cost is high and waste material is generated. On the other hand, with regard to the inkjet system, the equipment is inexpensive and, since an image is formed directly on a recording medium by discharging an ink only on a required image area, the ink can be used efficiently and the running cost is low. Furthermore, there is little noise and it is excellent as an image recording system.
An ink composition that can be cured by exposure to radiation such as ultraviolet rays and, in particular, an inkjet recording ink (radiation curing type inkjet recording ink) are required to have sufficiently high sensitivity and provide a high image quality. By achieving higher sensitivity, a large number of benefits are provided, such as high curability toward radiation, a reduction in power consumption, longer lifetime due to a decrease in the load on a radiation generator, and prevention of formation of low molecular weight material originating from insufficient curing. Furthermore, higher sensitivity particularly improves the cure strength of an image formed using the ink composition and, in particular, the inkjet recording ink, particularly for the formation of a lithographic printing plate, and high plate life can be obtained.
Conventionally, acids such as onium compounds, or radical polymerization initiators or cationic polymerization initiators, have hardly any adsorption at wavelengths longer than 360 nm and, when light exposure is carried out using a light source having a long wavelength, the curability is considerably reduced. In particular, since photopolymerization initiators such as sulfonium salts or iodonium salts, which are currently used, have hardly any absorption of UV rays on the long wavelength side, in the case of a thick film there is the problem that the curability becomes poor. Furthermore, in a photocuring composition containing a large amount of a white pigment such as titanium oxide having an absorption region in the ultraviolet (UV) region, there are many problems in practical use.
As a method for solving these problems, the use of a sensitizer has been examined; for example, it is reported that various types of dye derivatives are effective as a sensitizer for a diaryliodonium salt, and thioxanthone derivatives are known as particularly inexpensive and common sensitizers (JP-A-11-263804 (JP-A denotes a Japanese unexamined patent application publication)). However, there are the problems that the photocurability of a photocurable composition employing an additive such as a pigment is insufficient if only a thioxanthone derivative is used, and the stability over time is poor when an iodonium salt is used.
Photopolymerizable compositions using as a sensitizer an anthracene compound (JP-A-11-199681) or a 9,10-dialkoxyanthracene derivative (Japanese registered patent No. 3437069) have been examined. However, anthracene compounds and thioxanthone compounds have the problem that, since they have absorption up to long wavelengths, they develop a yellow coloration.